X-ray Variability of AGN

Judith Racusin

Astro 597A

Week 5

October 6, 2004

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Outline

• Timing Variability– PSD– RMS– GBH/BHXRB analogy to AGN– Time lags in UV/Optical

• Spectral Variability– Fvar

– Lines– Continuum– Models

• Conclusions

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Timing Variability

• A defining characteristic of AGN• Time scale of months to years, suggests scale of

emitting phenomena (one of the first clues of AGN BH connection)

• No obvious periodic behavior in light curves• Variable behavior on large range of time scales• McHardy et al. (2004) present observations

using RXTE and XMM-Newton of AGN (NGC 4051) & GBH(Cyg X-1 ) comparing various properties of these object mostly related to their PSDs

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Power Spectral Density (PSD)

• Variability can be characterized by PSD functions to understand the behavior

• “PSD defines the amount of variability ‘power’ as a function of temporal frequency” 1

• PSD ~ |Discrete Fourier Transform|2

• P f -α

5 of 33Vaughan et al. (2003)

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What does the PSD tell us?

• Break timescale– Scales with mass– Breaks from index of ~2 to

~1– Needed to avoid

divergence

• Amplitude of PSD in ~day-long light curves 1/Lx– Lx MBH

– tvar BH size– This works only for BLS1

not NLS1

Uttley et al. (2004)

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Long/Short Term Variability

• AGN with different luminosities are very different on short time-scales, but similar on long time-scales

• Length of observations may not spread across all long-term variability

• AGN appear to be “red noise” dominated (i.e. vary more on long time scales)

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PSD of NGC 4051

Fit by unbroken power law over frequency range 7x10-9 to 2x10-5 Hz has slope of -1.05

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The AGN-BHXRB Connection

• Power law shaped PSDs are similar to the “high” state PSDs of black hole X-ray binary systems (BHXRB)

• If BH mass scales linearly with break timescale, AGN should have break timescales of days-weeks

• This all assumes that the same phenomena is indeed acting on both kinds of systems

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“Low” & “High” States

• “high” state => high luminosity/soft x-ray spectrum

• “low” state => low luminosity/hard x-ray spectrum

• Distinguished by second break in low state PSD where slope = 0 about a decade below the high-frequency break

• High state seems to match some AGN well, but low state does not

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NGC 4051 & Cyg X-1

• NGC 4051 has best studied AGN PSD

• Cyg X-1 is a well studied BHXRB

• NGC 4051 PSD has same shape as Cyg X-1 PSD in the high state, scaled to higher frequency.

PSD has slope of ~ -1 at low frequencies and steepens to ~ -2

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Are AGN & BHXRB similar enough to use one to study the other?

• Similar PSDs that seem to scale with mass• Evidence from the rms-flux relation observed in

BH and neutron star XRBs suggest that variability originates in the accretion flow itself (i.e. not caused by coronal flares)

• Note (kind of obvious now): Assuming we believe that BHXRBs have BHs in them, this provides indirect proof that AGN have BHs. This is because they both show PSD breaks where as XRBs (neutron stars) do not but GBH (galactic black hole) candidates do.

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Physical Interpretations

• X-ray emission is produced by Comptonisation of low energy seed photons by energetic electrons in a corona

• The break timescale may correspond to the viscous or thermal time scales of ~few RG

• The non-flat slope of the PSD above the break suggests that the corona in not of uniform temperature, or fluctuations within the corona are responsible for the variability at high frequencies

• Additional parameters needed to explain variance between high and low state PSDs: perhaps accretion rate or BH spin (either would change edge of accretion region depending on relation to last stable orbit)

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Models to Account for PSDs

• Varying the temperature or density of the emitting region associates different photon energies with different radial locations– Accretion flow propagates inwards in an optically thin

corona over the surface of the disc until it hits the X-ray emitting region

– Higher energies and shorter timescales occur at smaller radii

– Characteristic timescale perhaps relates to viscous timescale at edge of X-ray emitting region

– Period dependant lags (lower frequencies propagate slower)

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RMS Variability

• RMS variability is measured for small segments of X-ray light curves in a specific band

• Mean and variance of segment calculated and then rms = sqrt(variance) in each bin

• Both AGN & BHXRBs show strong linear correlation between RMS amplitude of variability and X-ray flux

• Conclusion: more variance when object is brighter

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Time Lags• Time-scale dependent lags

between hard and soft X-ray• Optical/UV emission suffers from

slight lag to X-ray emission – suggests same mechanism as X-ray emission but reprocessed in the disk

• Lags are greater for longer Fourier period and increase with the energy separation between bands

• Lags are also asymmetric towards positive lags indicating the presence of complex delays of higher energy band lightcurves compared to lower energy

• Therefore, higher energies and higher frequencies are associated with smaller radii

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Optical Variability

• Long time scales require long, well-sampled monitoring campaigns

• Optical variability also red noise• Smaller amplitude of variability than in X-ray• Reverberation mapping from varying of optical

emission lines in response to continuum variations– Maps line emitting regions– Gives mass estimates

• Continuum emission thought to be primarily thermal from the accretion disk

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Optical/X-ray Correlations

• Some AGN show strong X-ray/Optical correlations, others do not

• Optical flux is at times larger than X-ray flux, ruling out optical source being pure reprocessing

• Why could this be?

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Cause of Optical/X-ray Variability• X-rays probably produced in optically thin

material close to BH independent of BH mass• Optical emission probably produced in optically

thick material via viscous dissipation and reprocessing– Radius of emitting region depends on BH mass,

because thermal (Temp Mass-1/4)– Emission also scales with accretion rate

• High accretion rate with low mass causes optical emission far from BH

• Low accretion rate with high mass causes optical emission closer to BH (i.e. closer to X-ray emission)

• Adding reprocessing and Compton cooling leads to range of variability seen

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Not Simple Absorption

• RXTE observations suggest that the X-ray variability cannot be caused by variable absorption from looking at the time averaged spectra– Changes in intrinsic luminosity would alter the

ionization states of the warm gas in the line of sight, thus altering the spectrum which is not seen

– At least between 2-10keV, lines of C, O, N, & Ne, do not appear to change

• Absorption would likely not affect such wide spectra range across different components of the AGN

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Spectral Variability

• Markowitz et al. (2003) present the broadband spectral variability of 7 Seyfert 1 galaxies observed with RXTE on time scales of days to years

• No evidence for simple correlation between continuum and spectral lines found

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Continuum Variability

• Source of continuum radiation is primarily thermal

• Likely source is an inner disk surrounded by a hot optically-thin corona that Compton-upscatters soft photons

• Continuums appear to vary more strongly towards longer time scales consistent with red noise

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Fe Kα (Line) Variability

• Source of Fe Kα emission is from an accretion disk that reprocesses some of X-rays (at 6.4keV)

• Fe Kα varies rapidly in all sampled AGN, in most, line and continuum variability is uncorrelated

• In some sampled AGN, core or wing of line varied more in response to continuum

• Time resolved spectral fitting shows wide range of variability on both long and short timescales but appears to be on average more red noise dominated

• Line flux varies by a factor of ~2 for all observations, well above the errors

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Fractional Variability Amplitudes (Fvar)

• Fvar is the intrinsic variability amplitude relative to the mean count rate and in excess of measurement noise

• Fvar = √((S2-<σ2err>)/<X>2), where S2 is the total

variance of the light curve, <σ2err> is the mean

square error, and <X> is the mean count rate

• Fvar spectra (Fvar of the net count rate in binned channels) confirms the relative continuum variability amplitudes are consistent with PSDs

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Measured Fvar

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Models/Mechanisms

• In simplest models: line formation depends on location in accretion disk– If emission originates close to BH, line is expected to respond

rapidly to changes in illuminating continuum flux– Should have time delay equal to that of light crossing time

• Laor Model (fit better than Gaussian)– Diskline model for a maximally rotating Kerr BH– Reasonable fits with a power-law component plus a

parameterization for Fe Kα

• Simple Model– Bulk of continuum originates in central corona and line variations

are driven by continuum flux variations scaled by light-travel time effects

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Variations to Simple Model• If bulk of line emission originates far from corona (BL region),

then continuum and line would be correlated only on large timescales– May take care of NGC 4151 and NGC 5548– Would suggest general trend of increase in number or

strength of continuum-line correlation on long time scales• Line emission in inner portions of disk is absent or

suppressed perhaps due to accretion disk maybe truncated, highly ionized, or radiatively inefficient

• Geometry of disk (being concave) could reprocess larger fraction of continuum photons in outer regions – reduces direct response between line peak and rapid continuum variations

• Flares that co-rotate at some height above the disk causing complicated fluctuations in continuum and line emission

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More Variations to the Simple Model

• Light-bending near BH could cause substantial de-coupling of continuum and line fluxes -- large changes in amount of continuum and line but little changes in total flux

• Ionized skin on surface of disk caused by thermal instabilities with most of neutral line emission originating in cooler layers beneath skin -- skin causes scattering from continuum and line sources removing correlations

• Ionized wind near the site of reflection would mimic an ionized skin

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Broadband Spectral Variability

• Two component model– Most of sampled AGN show strong correlation with no

measurable lag between 2-10keV continuum and photon index– Superposition of soft variable power-law likely associated with

coronal emission and a harder less variable spectral component associated with Compton reflection hump

– Intrinsic slope does not vary significantly, only relative contributions of components

• Broadband emission steepens as it brightens• Broadband spectral shape changes little for soft-

spectrum sources (NLS1) where as it varies more in hard-spectrum sources (BLS1)

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Conclusions

• PSDs of AGN and GBH are very similar in the high state

• These similarities lead to better understanding of the variability mechanism

• Variability is larger on longer time scales• Variability amplitudes increase towards softer

energies• There is no simple model that can explain all of

the correlated and uncorrelated variability in the line and continuum spectral components

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References

• 1) Vaughan, S., Edelson, R., Warwick, R.S., & Uttley, P. 2003, MNRAS, 345, 1271

• 2) Uttley, P., & McHardy, I.M. 2004, astro-ph/0402407

• 3) McHardy, I.M., Papadakis, I.E., Uttley, P., Page, M.J., & Mason, K.O. 2003, MNRAS, 348, 783

• 4) Markowitz, A. Edelson, R., Vaughan, S. 2003, AJ, 598, 935, 955